Extreme-ultraviolet (EUV) light is an electromagnetic radiation, e.g., having a wavelength range lower than about 100 nanometers or less. EUV light has been used in photolithography operation as an exposure light source to fabricate extremely small features.
The EUV light can be formed along with plasma by irradiating a target material with a laser beam, and the generated EUV light is then collected and transmitted to an exposure system by an optical device. During generation of the plasma and EUV light, however, debris of the target material and plasma by-product such as ions of the target material tend to deposit on the optical device, thereby damaging the optical device and deteriorating its optical performance.